A conventional analog TCXO has a thermistor network and a variable reactance diode, in which the thermistor network varies a voltage applied to the variable reactance diode as a prevailing temperature varies, and then the capacitance of the variable reactance diode varies. In other words, if the thermistor network senses any variation in the temperature, then the capacitance of the variable reactance diode varies to compensate a frequency deviation in a resonant frequency due to variation in the temperature.
The analog TCXO, however, can not keep so precisely the resonant frequency and can not be implemented in a integrated circuit.
As it is needed to keep the resonant frequency more precisely, a digital TCXO as shown in FIG. 1 was developed.
The digital TCXO consists of a temperature sensor 11, an analog-to-digital (AD) converter 12, a digital memory 13, a digital-to-analog (DA) converter 14, a variable reactance diode 15 and a crystal oscillator 16. The temperature sensor 11 is preferably formed in a temperature sensing circuit.
The temperature sensor 11 operates to sense a surrounding temperature of the crystal oscillator 16 and to output an electric signal corresponding to the surrounding temperature. The electric signal outputted in analog form is converted to a digital signal by the AD converter 12 and provided to the digital memory 13. The digital signal is used as address information of the digital memory 13. From the corresponding address of the digital memory 13, a capacitor array control code used in compensating a frequency deviations in the resonant frequency at the surrounding temperature is obtained. In the digital memory 13, voltage values applied to the variable reactance diode 15 to compensate frequency deviations in the resonant frequency due to variation of the surrounding temperature is recorded in the form of digital codes.
The capacitor array control code obtained from digital memory 13 is converted into an analog voltage level by the DA converter 14 and provided to the variable reactance diode 15. The variable reactance diode 15 varies its capacitance to compensate a frequency deviation in the resonant frequency of the crystal oscillator 16 due to variation of the surrounding temperature.
The digital TCXO can precisely keep the resonant frequency although the surrounding temperature varies. However, the variable reactance diode used in the digital TCXO prevents the digital TCXO from being implemented in an integrated circuit.
Another digital TCXO developed to solve this problem in the digital TCXO having the variable reactance diode is shown in FIG. 2. In this digital TCXO, therefore, a DA converter and a variable reactance diode are replaced with a capacitor array as shown in FIG. 3.
The capacitor array is formed of a plurality of unit cells 30. Each unit cell has a unit capacitor 31 and a switching element 32. One node of each unit capacitor 31 is connected to a crystal oscillator. Therefore, the load capacitance of the crystal oscillator is defined by combination of capacitances of all of the unit capacitors. The switching element 32 is inserted between the other node of the unit capacitor and an earth. Operation of the switching element 32 of each unit cell 30 is controlled by a control code provided from a memory.
Capacitances of the unit capacitors have either a constant value or different values, respectively. It is also possible that capacitances of only some other than all of the unit capacitors have a constant value.
In order to precisely compensate a frequency deviation in a large gradient region of a compensation curve shown in FIG. 4, a unit capacitor having a very small capacitance value is required. In a small gradient region, for example, in a region between Point A and Point B on the compensation curve shown in FIG. 4, however, it is not required to finely divide capacitance values obtained by combining capacitances of the unit capacitors. Thus, using unit capacitors having a constant value that is inevitably a very small capacitance value is not so desirable since a capacitance value required to compensate a frequency deviation in a small gradient region of a compensation curve is obtained by combining capacitances of an extravagant number of unit capacitors. Increasing the number of unit capacitors results in increasing the number of switching elements, increasing the number of bits constituting the control code, complicating a decoding logic, increasing an area required to connecting the unit cells with each other, and finally increasing the area of a silicon chip used in producing the capacitor array.
In order to reduce the silicon area used in producing the unit capacitors, unit capacitors having different capacitances, preferably increased by doubling, are used. Although decreased the number of unit cells and the silicon area used in the capacity array formed of unit capacitors having different capacitances, there is a problem of non-monotonicity.
In the capacitor array formed of a series of unit capacitors having capacitances increased by double of an immediately lower capacitance, an actual capacitance of a unit capacitor may be smaller than an ideal capacitance of the unit capacitor due to an inevitable tolerance in a semiconductor manufacture process. If the difference between the actual capacitance and the ideal capacitance of the unit capacitor is larger than an actual capacitance of another unit capacitor defined to have a lower ideal capacitance, a total load capacitance is decreased rather than increased when a unit capacitor is switched on while another unit capacitors defined to have a lower ideal capacitances are switched off by increasing the value of the control code. "Non-monotonicity" means the phenomenon that the load capacitance is decreased when the value of the control code is increased. In order to avoid the non-monotonicity, the minimum capacitance of a unit capacitor is limited over a reasonable capacitance. This limitation results in increasing a total capacitance used in temperature-compensating, and in turn increasing electric power consumption.